When designing structures of composite materials, the optimum material with regard to strength, weight, E-modulus and cost etc. is often not the same for all parts of the structure. For example, in a spar for a wind turbine blade the preferred material for the base part may be a glass fibre reinforced composite due to low cost and limited mechanical requirements whereas the preferred material for a load bearing outer part like a flange may be carbon fibre reinforced composite due to the higher stiffness and lower weight The physical properties, for example stiffness and thermal expansion, are very different, however, and it is the general conception in the art that such parts cannot be effectively connected.